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 The findings come from a study of the devastating genetic damage, or
mutations, caused by smoking in various organs in the body.
Publishing in the journal Science on Thursday, the researchers said
the findings show a direct link between the number of cigarettes
smoked in a lifetime and the number of mutations in the DNA of
cancerous tumors.
The highest mutation rates were seen in lung cancers, but tumors in
other parts of the body - including the bladder, liver and throat -
also had smoking-associated mutations, they said. This explains why
smoking also causes many other types of cancer beside lung cancer.
Smoking kills six million people a year worldwide and, if current
trends continue, the World Health Organization predicts more than 1
billion tobacco-related deaths this century.
Cancer is caused by mutations in the DNA of a cell. Smoking has been
linked with at least 17 types of cancer, but until now scientists
were not clear on the mechanisms behind many of them.
Ludmil Alexandrov of Los Alamos National Laboratory in the United
States, one of those who carried out the research, explained that in
particular, it had until now been difficult to explain how smoking
increases the risk of cancer in parts of the body that don't come
into direct contact with smoke.
"Before now, we had a large body of epidemiological evidence linking
smoking with cancer, but now we can actually observe and quantify
the molecular changes in the DNA," he said.
This study analyzed over 5,000 tumors, comparing cancers from
smokers with those from people who had never smoked.
It found certain molecular fingerprints of DNA damage – called
mutational signatures – in the smokers' DNA, and the scientists
counted how many of these were in different tumors.
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In lung cells, they found that on average, smoking a pack of
cigarettes a day led to 150 mutations in each cell every year. Each
mutation is a potential start point for a "cascade of genetic
damage" that can eventually lead to cancer, they said.
The results also showed that a smoking a pack of cigarettes a day
led to an average 97 mutations in each cell in the larynx, 39
mutations for the pharynx, 23 for the mouth, 18 for the bladder, and
six mutations in every cell of the liver each year.
Mike Stratton, who co-led the work at Britain's Wellcome Trust
Sanger Institute, said it was a bit like digging in to the
archaeology of each tumor
"The genome of every cancer provides a kind of archaeological
record, written in the DNA code itself, of the exposures that caused
the mutations," he said. "Looking in the DNA of cancers can provide
provocative new clues to how (they) develop and thus, potentially,
how they can be prevented."
(Reporting by Kate Kelland)
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